Cornstarch/polycaprolactone (SPCL) blends were prepared and used as external carbon source for biological denitrification in a constructed wetland. The denitrification performances, components of dissolved organic matter (DOM) and microbial diversity were investigated. The results showed that nitrate was removed mainly in the layer filled with SPCL, and the average denitrification rate was 0.069 kg/m(3) d (nitrate removal efficiency was 98.23%). The major component of DOM was polysaccharides which mainly consisted of reducing sugar. Besides, the concentrations of polysaccharides and reducing sugar decreased along the height of the constructed wetland. Therefore, the dissolved organic carbon (DOC) of effluent decreased to 6.54 mg/L. Denitrifying bacteria Bacillus (24.25%) and Thauera (9.36%) were the most abundant genera in the biofilm attached on the surface of SPCL. (C) 2014 Elsevier Ltd. All rights reserved.